Re: Proposal: Functor and friends for the wrappers in Data.Monoid

Thanks! On Sun, Feb 22, 2015 at 7:40 AM, Oleg Grenrus wrote:

And there are the ticket and the patch:

https://ghc.haskell.org/trac/ghc/ticket/10107 https://phabricator.haskell.org/D673

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On 22 Feb 2015, at 12:07, Oleg Grenrus wrote:

To clarify the discussion:

Let's add the following instances:

A: Functor, Foldable, Traversable to Sum, Product, Dual, First, and Last B: Data to everything in Data.Monoid (except Endo): Sum, Product, Dual, First and Last, All, Any, Alt C: Applicative, Monad, MonadFix to Sum, Product, Dual, First and Last D: Data to Identity

All seem to agree on A and B, C is "not sure", and D is new item.

- Oleg

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On 21 Feb 2015, at 17:34, Oleg Grenrus wrote:

I propose to add Functor, Applicative, Monad, Foldable, and Traversable and maybe even MonadFix instances to wrapper newtypes in the Data.Monoid module. The same way as in the semigroups package, e.g. https://hackage.haskell.org/package/semigroups-0.16.1/docs/Data-Semigroup.ht... < https://hackage.haskell.org/package/semigroups-0.16.1/docs/Data-Semigroup.ht...

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Basically:

instance Functor Sum where fmap f (Sum x) = Sum (f x)

instance Foldable Sum where foldMap f (Sum a) = f a

instance Traversable Sum where traverse f (Sum a) = Sum <$> f a

instance Applicative Sum where pure = Sum a <* _ = a _ *> a = a Sum f <*> Sum x = Sum (f x)

instance Monad Sum where return = Sum _ >> a = a Sum a >>= f = f a

instance MonadFix Sum where mfix f = fix (f . getSum) _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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